Detection apparatus



Dec. 5, 1950 H. ANDERsoN 2,532,874

DETEGTION APPARATUS Filed Dec. 13, 1945 4 Sheets-Sheet 1 Dec. 5, 1950 H. l.. ANDERSON 2,532,874

DETECTION APPARATUS Filed Dec. 1s, 1945 y 4 sheets-sheet 2 Dec. 5, 1950 H. l.. ANDERSON 2,532,874

nETEcTIoN APPARATUS Dec. 5, 1950 H. l.. ANDERSQN 2,532,874

DETECTION APPARATUS Filed Dec. l5, 1945 4 Sheets-Sheet 4 Patented Dec. 5, 1950 DETECTION APPARATUS Herbert L. Anderson, Hartford, Conn., assigner to the United States of America as represented by the United States Atomic Energy Commission Application December 13, 1945, Serial No. 634,862

26 Claims.

The present invention relates to an improved (Cl. Z50-27.5)

apparatusV for n'then detection and counting of neutrons.

In certain physical procedures as, for example, the measurement of the number of neutrons yielded in nuclear reactions oi the am type, during which nucleii bombarded by high energy alpha particles incorporate such particles into their nuclear structure and simultaneously eject a neutron, an instrument of very high neutron il detecting and counting eiiiciency must be used. The apparatus heretofore available in the art does not possess the sensitivity and accuracy required ior this purpose, and has proven unsatisfactory. The principal object of the present invention, therefore, is to provide an improved, highly sensitive means for accomplishing the detection and numerical counting oi neutrons, and particularly, the neutrons which are liberated as an incident to am, nuclear reactions.

As will hereinafter appear, the object of the invention is accomplished by the provision of a neutron detection and counting system of novel design, which includes an improved ionization chamber structure together with an improved combination alpha particle source and holder for the sample under test. The various features and advantages of the invention will be made more apparent in the accompanying drawings and in the following description oi one preferred embodiment thereof. In the drawings:

Fig. 1 is a section-al View through an ionization chamber constructed in accordance with the present invention, a diagrammatic representation of the electrical circuit of the complete apparatus being shown in operative connection therewith;

Figs. 2, 3, and 4 are, respectively, sectional views on the lines 2 2, 3 3, and 3--4 of Fig. 1;

Fig. 5 is a sectional view showing a preferred arrangement of the ionization chamber, a neutron reecting and moderating structure used in connection with the chamber, and certain oi the other structural elements of the system of the invention;

Fig. 6 is an enlarged, sectional View of the combination alpha particle source and sample holder of the invention; and

Figs. 7 and 8 are, respectively, section views in the neighborhood of the ionization chamber; and the electronic units, which include a preamplier I1, a linear amplifier and counter I9, and a high voltage D.C. supply 2l for the ionization chamber Il. The output from the ionization chamber I I is fed into the preamplier I1 which is desirably located closely adjacent the chamber, and the output from the preamplifier I1 is conducted to the input side of the linear amplifier I9, as shown in Figure 1.

The ionization chamber I I includes a generally cup-shaped main envelope or outer Vclosure 23, which is preferably in the form of a right cylinder and which is provided with a relatively deep, cylindrical. reentrant portion 25 at the closed end thereof. This outer closure 23 should be made of a material which is permeable to neutron radiation, and in order to simplify the design of the chamber, it may be made of a conducting material, such as aluminum. When made of conducting material, the outer closure 23 is utilized as the outer electrode of the ion chamber. The inner electrode 21 is of cup-shape, and is supported within the outer electrode or closure 23 so as to surround the cylindrical, reentrant portion 25 with substantially uniform .spacing therebetween.

The support means for the inner electrode 21 comprises a multiple glass-to-metal seal structure which includes two cup-shaped members 29 and 3| of metal, a tubular member 33 also of metal, and two tubular sections 35 oi glass. The several members comprising the inner electrode support means are of substantially the same diameter, and are joined together by four glass-to-metal seals 31, as shown in Fig. 1. The upper end of the cup-shaped metal member 29 is conductively and mechanically attached to the lower end of the electrode 21. The other cup-shaped metal member 3l supports an elongated tubular member 39, preferably cylingdrical in form, which is adapted to enclose an insulated lead-in conductor 4I to the inner electrode 21 through the metal member 29. The tubular member 39 is -insulated from the other portion of the ion chamber by the tubular insulators 35, and it is normally grounded, thereby serving as a guard or shield ring for the lead-in conductor GI, as is conventional in ionization chamber construction. The tubular on the lines 1--1 and 8-8 of Fig. 6. 50

Generally stated, the neutron detection and conducting member 33, which comprises the counting apparatus of the present invention inlower end of the seal structure, is welded to a cludes four main elements. These are: the cup-shaped support 43 which in turn is xedly ionization chamber II, which is illustrated parattached to an annular plate member 45. This ticularly in Figs. 1 to ll; the combination alpha 55 plate member 45 is bolted to the lower end of particle source and sample support I3, which the main closure 23 by a series of symmetrically is illustrated particularly in Figs. 6, '1, and 8; the spaced cap screws 41. The lower end of the neutron reflecting and moderating structure I5, main closure 23 is thickened, as illustrated at illustrated in Fig. 5, which serves as a means 49, to accommodate the cap screws t1; and the for slowing the neutrons to be detected to low 50 engaging surfaces of the closure 23 and the plate energies and for maintaining their density high.

45 are provided with interengaging parts 5I 3 which, in combination with a suitable gasket 53, provide a, gas-tight seal for the closure, while at the same time permitting easy disassembly of the structure.

During use it is desired that the ion chamber II shall be lled with boron triiiuoride (BFs) at a pressure of about 80 cms. of mercury, this material having the property of becoming ionized when subjected to neutron irradiation. While normal boron tri-luoride is satisfactory the sensitivity of the device can be considerably enhanced by the use of boron triuoride which is enriched in the isotope of boron of mass ten, B10. The desired enrichment can be conveniently accomplished by mixing boron trifluoride made from the -isotope B10 and normal boron triuoride. If maximum sensitivity is desired the B10 gas may be used without dilution.

'To facilitate the introduction of the neutronresponsive gas into the ionization chamber, thc device includes a suitable valve 55, which is mounted in the annular plate member 45 and which communicates with the interior of the device through a passageway 57. The plate member 45 also includes a centrally disposed opening 59 of sufficient size to clear the multiple seal support for the inner electrode 21.

The combination alpha particle source and sample holder I3, which is illustrated in Figs. 6, 7', 'and 8, includes a cylindrical, cup-shaped, outer casing 6I which is closed at its lower end by a circular plate member 63 having an inwardly projecting' boss or platform 65 formed therein. The plate vmember 63 is fastened in place by screws 6d, as illustrated, and the engaging surfaces of the plate member 63 and the casing 6I desirably include intertting parts 61 for engaging a gasket 69 to provide a gas-tight seal. The sample of the material which is to be subjected to alpha particle irradiation is supported upon the inner face of the platform 65 by any convenient means. A nat, circular sample 'II adapted to be held in place by a cupshaped holder 73 is illustrated in the drawings,

but it will be understood that the sample might be of a diiferent shape, or might be a powder or liquid, etc.

The source of alpha particle radiation used in this particular device comprises a circular plate v'I5 of polonium (P0210) which is supported upon an annular support member 'I1 directly above, and in line with, the sample platform 6,5. The isotopes of polonium, as is known, have the property of emitting alpha particle radiation during the normal decay of such isotopes to more stable neucleii. The 210 isotope exhibits a half-life characteristic of alpha particle emission of about 136 days which particularly adapts i-t for use as an alpha particle source. In v'order to extend the range of the device, it is desirable that means be available whereby `the source of alpha radiation can be located at various. dis- -tances from the sample under test. This is conveniently accomplished by the use of a micrometer screw support 79 which is connected to the source holder 'Il by a rod member 8|. By this means, the source plate 'I5 may be moved toward or away from the sample 1I which -is supported upon the platform 65, and the micromthe structure illustrated communicating with the interior of the closure through a tubular member 8 5..

The dimensions of the combination source and sample holder means I3 should be such that the device will t within the cylindrical depression provided by the reentrant portion 25 at the end of the ionization chamber closure 23, and it is shown in this position in Figs. 1 and 6.

It will be apparent that with the structure described, the ions formed within the ion chamber I I, as a result of the irradiation of the boron triluoride contained within that chamber by the neutrons liberated due to the alpha particle bombardment of the sample material, have a very `short path of movement within the chamber. For this reason most of the pulses produced by the neutron disintegration of boron have the same size; on the other hand the background pulses which come from alpha particle contamination emitted from the walls of the chamber have different sizes. The associated electronic equipment is designed to discriminate in favor of the pulses produced by boron disintegration `and record those preferably over the pulses of different size. This improves the ratio of neutron produced pulses over those of the background.

'Ihe operating circuit, as previously stated, is illustrated diagrammatically in Fig. 1. The guard ring shield member 39, one side of the input to the preamplifier I'I, and the negative side of the high voltage direct current supply 2I are grounded. The positive side of the high voltage direct current supply is connected to the outer casing 23 which thus serves as the high voltage electrode. The inner ion chamber electrode 21 is connected to the other side of the preamplifier' input, and the preamplifier output is fed directly to the linear amplier and counter 19. The preampliger I'I, the linearV amplifier and counter I9. and the high voltage direct current supply 2 I may all comprise electronic systems of known design, and do not constitute -a part of the present invention.

previously stated the, ionization chamber I I is enclosed within a neutron reilecting and moderating structure, as illustrated at I5 (Fig. 5.). The structure,l I5 preferably comprises at -le'astsix inches of hydrogenous material, such as paraflin, although other known neutron slowingor moderating lmaterials suchasY heavy water, D20', carbon, or beryllium can be used at some sacrifice to the sensitivity of the device. The structure I5 has the functions of slowing the neutrons passing into and through the ionization chamber from the sample, thereby assuring that substantially all such neutrons will be slowed to detectable energies, andA -of reeoting or directing the slowed neutrons into the ionization chamber so as to maintain 4maximum neu-- tron density in the-.region of the chamber II.

'Paraflin has an additional advantage in that it is a good 'electrical insulator, and the structure illustrated at I5 is of that material. Thestructure I5. is in two parts 8:7 and 85. The. inner part 3:1` is cylindrical inform and is conveniently made by casting a block of .parain about the ionization chamber Il and the upper end of the vpreamplilier I 'I as isv illustrated in Fig. 5. An opening 9iis provided to permit access to the .reentrant portion 25 in order that the source and sample holder I3 .may be .put in place, and the complete unit comprising vportion 81, the

eter scale provides a continuous indication of the 7( relative position of the parts. It 'will usually b e found desirable to evacuate the interior of the source and sample holder means I3, and to this end, the outer closure 6I is provided with a valve '83 of any suitable design, the valve 83 in 75 ion chamber Ill., and thepreamplier I1 is adapted to slide into a suitable opening 93 provided in the outer section 89. The high voltage connection to the outer casing 23, which serves as the outer electrode of the ionization chamber II, is made via a conducting tube 95 which is held in place by the cast parain inner portion 81 and which extends through an opening 91 in the outer portion 89. The preamplier I'I is enclosed within a metallic shield 99 as illustrated. The shield' 99 is connected to the tubular guard ring member 39 by a suitable conductor lill, and both members .are grounded. A cast paraffin block H33 iits over the two shield portions 31 and 89, and block |63 is provided with a central opening m5, normally closed by a removable plug IU?, to permit access to the recessed upper end of the ionization chamber II. Desirably, the entire parafn block shield structure is reinforced by a suitable framework, as indicated at Hi8 and IH, and this framework may be supported upon resilient blocks H3 to minimize microphonic disturbances.

The very close spacing of the ionization chamber structure i3 and the preamplifier Il, made possible by the disclosed arrangement, permits the use of a very short connection H from the lead-in conductor 4i for the inner electrode 2l to the lrst tube IIl of the preamplifier. This further aids in increasing the efficiency of the system.

During the operation of the device, a sample of the material under test will be placed upon the sample support platform 65 within the conibination alpha particle source and sample holder I3. The ambient conditions within the holder I3 will be adjusted to those desired, either by evacuation of the interior of the device, or by the introduction of a gas thereinto. The position of the alpha particle source plate 'i5 relative to the sample will normally be adjusted by operation of the micrometer screw 'I9 when the measuring operation starts, although a preliminary adjustment may be made at this time.

The combination particle source and sample holder I3 is then introduced into the recess at the end of the ion chamber provided by the reentrant portion 25, and the shield plug IIlI is put in place. Upon energization of the ampliers and counter means I'I and i9, and the high Voltage direct current supply 2l, the system is ready for operation. Calibration may be effected by introducing samples of known neutron activity into the recess provided by the reentrant portion 25. In actual test, the apparatus has proven more eicient than known neutron detection and counting means. It is accurate, extremely sensitive, and has a low background effect being capable of measuring in a few hours the output from am nuclear reactions yielding as small an output as one neutron per second, a

.degree of sensitivity heretofore considered practically unattainable. The features of the invention which are believed to be new are set forth in the appended claims.

What is claimed is:

l. An ion chamber for use in connection with apparatus of the class described, comprising an outer closure of a conductive material which is permeable to neutron particle radiation, said closure serving as one of the electrodes of said ion chamber and having a reentrant portion Which defines a recess for containing a sample of material under test. and a generally cupshaped inner electrode supported within, and insulated from, said outer closure in suchposition that it surrounds said reentrant portion.

2. An ion chamber for use in connection with apparatus of the class described, comprising a sealed outer closure of a conductive material which is permeable to neutron particle radiation, said closure containing a gas which becomes ionized when subjected to neutron irradiation, said closure being adapted to serve as one of the electrodes of said ion chamber and having a reentrant portion which denes a recess for containing a sample of material under test, and a generally cup-shaped, inner electrode supported within, and insulated from, said outer closure in such position that it surrounds said reentrant portion.

' 3. An ion chamber for use in connection with apparatus of the class described, comprising a sealed outer closure of a conductive material which is permeable to neutron particle radiation, said closure containing a gas which becomes ionized when subjected to neutron irradiation, said closure being adapted to serve as one of the electrodes of said ion chamber and having a reentrant portion at one end thereof which defines a recess for containing a sample of material under test, a generally cup-shaped, inner electrode, and means including a glass-tometal seal :for supporting said inner electrode within, and for insulating said inner electrode from, said outer closure, said inner electrode being disposed in such position that it surrounds said reentrant portion, thereby minimizing the path of ion movement within said chamber.

4. An ion chamber for use in connection with apparatus of the class described, comprising a sealed, cylindrically shaped, outer closure of a conductive material which is permeable to neutron particle radiation, said closure serving as one of the electrodes of said ion chamber, having a cylindrical reentrant portion disposed centrally of one end thereof which defines a recess for containing a sample of material under test, and containing a gas which becomes ionized when subjected to neutron irradiation, .a cylindrical, cup-shaped. inner electrode, and means including a glass-to-metal seal for supporting said inner electrode within, and for insulating said inner electrode from, said outer closure, said inner electrode being supported concentrically of said reentrant portion and surrounding that portion, thereby minimizing the path of ion movement within said chamber.

5. An ion chamber for use in connection with apparatus of the class described, comprising a sealed, cylindrically shaped, outer closure o a conductive material which is permeable to neutron particle radiation, said closure serving as one of the electrodes of said ion chamber, having a cylindrical reentrant portion disposed centrally of one end thereof which defines a recess for containing a sample of material under test, and containing a gas which becomes ionized when subjected to neutron irradiation, a cylindrical, cup-shaped, inner electrode, a lead-in connection for said inner electrode, a tubular guard ring shield for said lead-in connection, and means including a multiple glass-to-metal seal structure which supports said inner electrode within, and which insulates said inner electrode from, said outer closure, said seal structure also supporting said guard ring shield in such position that it encloses said lead-in connection and insulating said guard ring shield from said outer closure, from said inner electrode, and from said lead-in connection, said seal means insulating boron ten.

8. An ion chamber `as definedin'claim Swhere- :in the Agas contained 1in said closure .comprises -boron trifluoride.

9. An ion chamber as denediinclaim 4wherein the gas contained in .said closure comprises boron 4trifluoride enriched -with fthe separated 'isotope boron ten.

10. In combination in apparatus -of the :class described, an ion chamber :comprising an ,outer closure of a conductive'material'which is permeable to neutron particle radiation, :said closure serving as one of the electrodesofrsaid'ion'chamber and having a reentrant portion rwhich defines Aa recess for containinga sample of material under test, a generally cup-shaped innerelectrodefsupported within, vand insulated from, said outeraclosure in such position that y-it `surrounds `said lieentrant portion, and a neutron .slowing and reecting means disposed about said ion chamber. 'i

11. In combination in apparatus -of the-class described, an ion chamber fcompr-ising v'a sealed outer closure of a conductive *material which is permeable to neutron particle Iradiation, =said=clo sure containing a gas which Ibecomes `ionized when subjected to neutron 'irradiationpsaidcl Vsure being adapted to serve as'one of the felectrodes of said 'ion chamber iand having a reentrant portion which "defines a recess .ifor 'containing a .sample yof lmaterial :under test, a :gen-

erally cup-shaped, inner electrcdesupport within,

and insulated-from saidvouterclosure in such -position that it'surrounds larltzl-anneiitron slowing and freflectingimeansof a --said 'reentrant portion,

-neutron moderating materialdisposed.` about.` said -ion chamber.

'12. In combination lin apparatus o'fthe class described, an ion chamber `'comprising La sealed outer closureof a conductivcm'aterial =which1is permeable-to neutron'particle radiation, saidfclo- -lsure'containing a gaslwhichlbecomes ionizedwhen subjected to neutron irradiation, 'sai-d closure 4'being adaptedto-serve as oneof theelectrodes of 4said ion chamber and-having areentrantrportion taining a sample of material under 'test,ra igenerally cup-shaped inner electrode, meanszincluding a glass-to-metal seal supporting said inner electrode within said outer closurein such positrifluoride -enriched with the separated isotope at one end thereof which defines-airecess for conf tion that it surrounds, -andlis substantially uniformly spaced from, said reentrant portion, said said inner 1 electro-de `from said outer closure, an' a neutronslowing `and reflecting means disposed aboutfsaidY ion chamber,

'13. In combination `in apparatus of the -class v"described, an ion-chamber comprising Aa sealed,

"cylindrically-shaped outer closure of a conductive material which is permeable-*to neutronparticle radiation, saidclosure servingas one of'V theele'c- "trodesvof said ion chamberjhaving a cylindrical f'reentrant portion A'disposed "centrally c'f one `end thereof which defines a-recessifor containing a sample of material under test, andfcontaining a gas which becomes ionized vWhensubjected to neutron lirra'dition, a cylindrical, Vcmi-shaped,

inner electrode, and means including ag-lass-to- `metal seal for .supporting said inner electrode within lsaid outer closure in such position that it surrounds said reentrant portion, said inner electrode being supported :concentrically of :said reentrant portion .and being insulated therefrom, 4'anda neutron vslowing and 4reflecting means =of :a hydrogenous 4neutron moderating material disposed :about said ion chamber.

14. combi-nation in apparatus of the class describedin an ionchambercomprising a sealed, Acylindrically shaped, `outer .closure of afconductive :material which lis Ipermeable to :neutron particle radiation, said closure serving as vone-of the :electrodes of said ion chamber, having -a cylindri- -cal reentrant portion disposed .centrally of one -end thereof which defines `a recess .for Vcontaining Aa sample oi Imaterial under test, and containing a gas which 'becomes ionized when subjected Ito neutron irradiation, a cylindrical, cup-shaped, inner-e1ectrode,=a -lead-in-connection for said inner electrode, a tubular guard ring shield for said 'lead-'in connection, and means vincluding a -mul- 'tiple, glass-to-metal, insulating, seal structure which Asupports said 'inner electrode -within said outer closure in `such `position that -said inner -eleotrode surrounds,'and Vis substantially uniformly spaced from, said reentra-nt portion, said seal means insulating said inner electrode from said outer closure and supporting said guard -ring shield in such manner that said shield encloses said lead-in connection, and-is-insulated'from said outer closure and fromrsaidiinner electrode-and said lead-in connection, and a neutron-slowing `'and reflecting -means on a lneutron :moderating material disposed about said ion chamber.

'15. Apparatus as defined iin claim 11 wherein the gas contained `insaidclosure comprises'lboron trifluoride.

16. Apparatus as -dened =inclaim `11 Aw-herein the gas-contained'in said closure comprisesboron trifluoride enriched with the separated isotope boron ten.

11.7. Apparatus as deiined in claim 1-2 wherein the gasrcontained in sadfclosure comprisesboron triuoride.

18. Apparatus as defineddn claim'12 -wherein theigascontained in said closure comprises boron .triuoride 4enriched with `the separated isotope boron ten.

19. In .-combinationin apparatus of the-class described, lan ion chamber comprising an outer closureof a conductive material which ieper- .meablerto-neutron particleiradiation, said closure serving as one. of the electrodes-of said ion cham- :iberiand having a. reentrantportion Whichideflnes :a relatively deep recess, .and a generally vcup- ;shapedinner electrodesupported within, andin- 4sulated from, said'outerclosurein such position :that :it surrounds said reentrant portion, 'and a -combinationalpha aparticle .source and sample holder disposed .within saldi-recess.

20. In combinationin'apparatuslof the-class described, an ion-chamber comprisingla sealed outer closure of a conductive 'material which is permeable to `neutron -particle radiation, said closure `containing l:a gas which becomes ionized when subjected to -neutron irradiation, said closure beingadapted to-serve as one of the electrodes of saidsionfchamber andr'having areenxtrantlvportion at one endzthereof which denes yarrelativelydeepfrecess, and a generally cupshapedinner electrode supported Within, and :insulated from, :said iouter .closure :in isuchfposition that.. itsurroundszzsaid reentrantportiomwith substantially uniform spacing therebetween, and a combination alpha particle source and sample holder disposed within said recess.

2l. In combination in apparatus of the class described, an ion chamber comprising a sealed, cylindrically shaped outer closure o1" a conductive material which is permeable to neutron particle radiation, said closure serving as one of the electrodes of said ion chamber, having a relatively deep, cylindrically shaped, reentrant portion which is disposed centrally of one end thereof and which defines a relatively deep recess, and containing a gas which becomes ionized when subjected to neutron irradiation, a cylindrical cup-shaped, inner electrode, and means includinga glass-to-metal seal forn supporting said inner electrode within, and for insulating said inner electrode from said outer closure, said inner electrode being supported concentrically of said reentrant portion and surrounding that portion with substantially uniform spacing therebetween, and a combination alpha particle source and sample holder disposed Within said recess.

22. In combination in apparatus of the class described, an ion chamber comprising a sealed, cylindrically shaped, outer closure of a conductive material which is permeable to neutron particle radiation, said closure serving as one of the electrodes of said ion chamber, having a relatively deep cylindrically shaped, reentrant portion disposed centrally of one end thereof, and containing a gas which becomes ionized when subjected to neutron irradiation, a cylindrical, cup-shaped inner electrode, a lead-in connection for said inner electrode, a tubular guard ring shield for said lead-in connection, and means including a multiple glass-to-metal seal structure which supports said inner electrode within, and which insulates said inner electrode from said outer closure, said seal structure also supporting said guard ring shield in such position that it encloses said lead-in connection and insulating said guard ring shield from said outer closure, from said inner electrode, and from said lead-in connection, said inner electrode being supported concentrically of said reentrant portion and surrounding that portion, and a combination alpha particle source and sample holder, which includes a cylindrical outer casing, fitting within said recess.

23. In combination in apparatus of the class described, an ion chamber comprising an outer closure of a conductive material which is permeable to neutron particle radiation, said closure serving as one of the electrodes of said ion chamber and having a reentrant portion which denes a relatively deep recess, and a generally cup-shaped inner electrode supported within, and insulated from, said outer closure in such position that it surrounds said reentrant portion, a combination of alpha particle source and sample holder disposed within said recess, and a neutron slowing and reilecting means disposed about said ion chamber.

24. In combination in apparatus of the class described, an ion chamber comprising a sealed outer closure of a conductive material which is permeable to neutron particle radiation, said closure containing a gas which becomes ionized when subjected to neutron irradiation, said closure being adapted to serve as one of the electrodes of said ion chamber and having a reentrant portion at one end thereof which denes a relatively deep recess, and a generally cup-shaped, inner electrode supported within, and insulated from, said outer closure in such position that it surrounds said reentrant portion with substantially uniform spacing therebetween, a combination alpha particle source and sample holder disposed Within said recess, and neutron slowing and reflecting means of a hydrogenous, neutron moderating material disposed about said ion chamber.

25. In combination in apparatus of the class described, an ion chamber comprising a sealed. cylindrically shaped outer closure of a conductive material which is permeable to neutron particle radiation, said closure serving as one of the electrodes of said ion chamber, having a relatively deep cylindrically shaped reentrantVY portion which is disposed centrally of one end thereof and which defines a relatively deep recess, and containing a gas which becomes ionized when subjected to neutron irradiation, a cylindrical cup-shaped inner electrode, means including a glass-to-metal seal for supporting said inner electrode within, and for insulating said inner electrode from said outer closure, said inner electrode being supported concentrically of said reentrant portion and surrounding that portion with substantially uniform spacing therebetween, a combination alpha particle source and sample holder disposed within said recess, and a neutron slowing and reflecting means disposed about said ion chamber.

26. In combination in apparatus of the class described, an ion chamber comprising a sealed, cylindrically shaped, outer closure of a conductive material which is permeable to neutron particle radiation, said closure serving as one of the electrodes of said ion chamber. having a relatively deep cylindrically shaped reentrant portion, disposed centrally of one end thereof, and containing a gas which becomes ionized when subjected to neutron irradiation, a cylindrical, cup-shaped, inner electrode, a lead-in connection for said inner electrode, a tubular, guard ring, shield for said lead-in connection, means including a multiple glass-to-metal seal structure which supports said inner electrode within, and which insulates said inner electrode from, said outer closure, said seal structure also supporting said guard ring shield in such position that it encloses said lead-in connection and insulating said guard ring shield from said outer closure, from said inner electrode, and from said lead-in connection, said inner electrode being supported concentrically of said reentrant portion and surrounding that portion,

'f a combination alpha particle source and sample holder, which includes a cylindrical outer casing, fitting within said recess, and a neutron slowing and reflecting means of a neutron moderating material disposed about said ion chamber.

HERBERT L. ANDERSON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Friedman Aug. 13, 1946 

